The links I read about UNDO tablespace / datafile management all mention adding datafiles, but none of them mention removing a datafile from UNDO. On one of my servers I do not have a lot of space, so I would like to add some space to UNDO, perform a task, and then remove the datafile again later.

My logic tells me that if you add a datafile to UNDO, Oracle will start using it, and when you want to drop it again, you won't be able to, as the datafile will be in use.

Is it possible to do this ?

I did not find anything about dropping a datafile in the Administrator's Guide, 10g Release 2 (10.2)

From the guide:
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Altering an Undo Tablespace
Undo tablespaces are altered using the ALTER TABLESPACE statement. However, since most aspects of undo tablespaces are system managed, you need only be concerned with the following actions:

■ Adding a datafile
■ Renaming a datafile
■ Bringing a datafile online or taking it offline
■ Beginning or ending an open backup on a datafile
■ Enabling and disabling undo retention guarantee

These are also the only attributes you are permitted to alter.
If an undo tablespace runs out of space, or you want to prevent it from doing so, you can add more files to it or resize existing datafiles.